Digital communication link monitoring device

Abstract

Digital transmission service offerings provided by common carriers are characterized by digital terminations at two or more user locations. The user has access only to the digital clock and data signals. A modem is described which when attached to the digital terminations provides three functions. First it allows the user to pass his data over the digital service channel. Second it measures the digital communication error rate and availability. Third it permits the user to pass a low speed telemetry channel over the digital service in addition to normal user data. The modem measures the digital channel bit error rate and provides a secondary or side channel without interfering with the transmission of normal data.

I claim: 
     
       1. A digital data communication system comprising: data terminal equipment provided at several locations;   a digital data communication media interconnecting said locations and adapted to carry digital data signals;   interfacing means provided between said data terminal equipment and said communication media; and   digital monitor units provided at said several locations and   connected between said interfacing means and said data terminal equipment, said digital monitor units being adapted to exchange monitoring data.   
     
     
       2. The system of claim 1 wherein said data terminal equipment at different locations exchange data streams and wherein said monitoring data is embedded in said data stream. 
     
     
       3. The system of claim 2 wherein said monitoring data comprises test signals, error rate data, and communication channel arbitrary data. 
     
     
       4. The system of claim 3 wherein said digital monitoring unit comprises an auxiliary processing element provided to calculate said error rate data corresponding to the test signals received from another digital monitoring unit. 
     
     
       5. The system of claim 4 wherein said auxiliary processing element is adapted to measure the performance of said communication media, and wherein said monitoring data further comprises said measurement. 
     
     
       6. The system of claim 5 further comprising a monitor and display means connected to at least one of said digital monitoring units for supervising said units. 
     
     
       7. The system of claim 2 wherein the rate of monitoring data is lower than the rate of data generated by said terminal equipment. 
     
     
       8. The system of claim 7 wherein said digital monitoring unit receives a first clock signal from the interfacing means corresponding to the transmitted rate of said interfacing means and wherein said digital monitoring means provides a second clock signal to said digital terminal equipment corresponding to the data rate transmitted by the digital terminal to said monitor unit, said first clock signal having a higher frequency than said second clock signal.